Luminescent material for a mercury discharge lamp

Abstract: A luminescent material (26) for a mercury discharge lamp (10) comprises a phosphor material including phosphor particles for emitting a luminous flux upon excitation by ultraviolet radiation at 254 nm and a protective layer continuously formed on the phosphor particle with at least one metal oxide selected from the group consisting of MgO, Y₂O₃, La₂O₃, Sm₂O₃, Gd₂O₃, Dy₂O₃, Ho₂O₃, Er₂O₃, Yb₂O₃, Lu₂O₃, CaO, ZrO₂, SrO, BaO, α -Al₂O₃ and BeO. A mercury discharge lamp has a luminescent layer coated with the luminescent material on the wall of the light transmissive bulb thereof.

Description: The present invention relates to a luminescent material for a mercury discharge lamp and more particularly to a luminescent material for a fluorescent lamp, which includes a phosphor material emitting a luminous flux upon excitation by ultraviolet radiation having a wave length of 254 nm. A mercury vapor discharge lamp, such as a fluorescent lamp is widely used as light sources for general illumination purposes, office automation machines, light-emitting elements of the giant screen display apparatus and back lights for liquid crystal displays. The fluorescent lamp has a luminescent layer composed of a phosphor and formed on an inner envelope of a glass tube in which mercury and one or more rare gases are filled. Electrodes are suitably arranged in a glass tube and a discharge is caused therebetween. Mercury in the glass tube is vaporized by the discharge and mainly ultraviolet radiation is produced. A comparatively small part of the ultraviolet radiation has a wavelength of 185 nm while the greatest part has a wavelength of 254 nm. Upon excitation of the phosphor by the ultraviolet radiation, a luminous flux or visible light is produced. The visible light is irradiated outside the glass tube. For general illumination purposes, a three band type fluorescent lamp, which efficiently generates white light and has excellent colour rendering properties, has become popular. The luminescent layer of the three band type fluorescent lamp is composed of blue, green and red emitting phosphors each having a relatively narrow band emission spectrum distribution. The fluorescent lamp mentioned above has a drawback because the luminous efficacy is reduced with increasing operating hours. In particular, reduction of luminous efficacy in a high wall loaded fluorescent lamp is considerably higher. As disclosed in Japanese Patent Disclosure (Kokoku) 3-45505, the reduction in the the luminous efficacy has been supposed to be caused by the following two factors. Firstly, the phosphor material itself deteriorates upon exposure to the ultraviolet radiation of wavelength 185 nm. Although the intensity ratio of ultraviolet radiation having the wavelength of 185 nm to that of the wavelength of 254 nm is about 10 to 12 %, it becomes more than 12 % when the fluorescent lamp is operated with a relatively high wall load as compared to the conventional fluorescent lamp. The lamp tube can be formed in various shapes like a circle, a U-shape, a saddle-shape and other complicated shapes in addition to the ordinary straight tube. As the fluorescent lamp has been made smaller in size, the wall load of the lamp increases. The phosphor material coated on the small lamp tube deteriorates and the luminous flux emitted from the phosphor material is lowered accordingly. Secondly, the reduction of the luminous efficacy is caused by blackening of the lamp partly or fully occurring on the inner wall of the lamp tube. The blackening is mainly due to the deposition of residues of organic binders and/or electrode substances, however the adhesion of mercury or compound(s) thereof to the phosphor material coated as the luminescent layer on the inner wall of the lamp tube is believed to cause the blackening. The reason why mercury is adhered to the phosphor material is supposedly by attractive forces exerted on mercury and the phosphor material. It is also believed that the phosphor material deteriorates by ion and/or atom bombardments. Japanese Patent Disclosure (Kokoku) 3-45505 discloses phosphor materials as preferred illuminating materials for fluorescent lamps operated with a relatively high wall load. The phosphor materials are claimed to deteriorate less under exposure to ultraviolet radiation at 185 nm and are claimed to be less reactive to mercury . However they are required to satisfy a property of having (at 254 nm-excitation) a luminous flux which, after the phosphor materials have been subjected for 15 minutes to ultraviolet radiation of a wavelength of mainly 185 nm and 254 nm and a radiation density between 150 and 500 W/m 2 and a ratio of 185 nm power between 0.20 and 0.40, is not more than 5 % lower than the initial luminous flux of the phosphor materials also at the 254 nm excitation and measured in identical circumstances. A combination of cations in the phosphor materials has an electronegativity of not more than 1.4. The present invention has for an object to provide an improved luminescent material containing a phosphor material for a fluorescent lamp which may be operated with a relatively high wall load wherein the improved luminescent material deteriorates less under ultraviolet radiation at 185 nm, which is preferably less reactive to mercury. Preferably, the phosphor material need not satisfy the above-described property disclosed in Japanese Kokoku 3-45505. According to a first aspect of the present invention, a luminescent material for a fluorescent lamp comprises a phosphor material including phosphor particles for emitting a luminous flux upon excitation by ultraviolet radiation having a wavelength of 254 nm and a protective layer continuously formed on the phosphor particle for substantially preventing ultraviolet at 185 nm from transmitting therethrough. The protective layer is composed of one or more metal oxides so as to prevent mercury from adhering the phosphor particles. For the protective layer, at least one metal oxide is selected from the group consisting of MgO, Y 2 O 3 , La 2 O 3 , Sm 2 O 3 , Gd 2 O 3 , Dy 2 O 3 , Ho 2 O 3 , Er 2 O 3 , Yb 2 O 3 , Lu 2 O 3 , CaO, ZrO 2 , SrO, BaO, α -Al 2 O 3 and BeO, such protective layer being obtainable by the process steps specified in claim 1. According to a second aspect of the present invention, in preferred embodiments, the metal oxide has an energy gap ranging from 5 electron volts to 8 electron volts at the forbidden band, which substantially corresponds to quantum energy of ultraviolet at 185 nm. According to a third aspect of the present invention, in preferred embodiments the metal oxide has a metal ion electronegativity Xi ranging from 5 to 9, wherein Xi = (1+2Z)X o in which Z represents the ion valence of the metal ion and X o denotes the Pauling's electronegativity. In order that the invention may be illustrated, more easily appreciated and readily carried into effect, embodiments thereof will now be described purely by way of example, with reference to the accompanying drawings, which constitute a part of the specification, and wherein: FIG. 1 is a schematic view, partly in section, of a fluorescent lamp constructed in accordance with the present invention; FIG. 2 is an enlarged cross sectional view of a luminescent material useful in fluorescent lamps in accordance with the present invention; FIG. 3 is an enlarged cross sectional view of a modified luminescent material useful in fluorescent lamps in accordance with the present invention; FIG. 4 is a graph showing the spectral transmittance of various metal oxides; and FIG. 5 is a schematic view of a lamp fixture mounting the fluorescent lamp shown in FIG. 1. DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS Referring now to FIG.1, a fluorescent lamp 10 has a tubular glass bulb 12 fabricated of soda-lime glass and mounts 14 sealed into either end thereof. Each mount 14 comprises a vitreous portion sealed to an end of the bulb 12 with lead-in conductors 16 sealed therethrough and supporting at their inwardly extending extremities refractory metal coils 18, which are fabricated of tungsten, for example. Contained within turns of these coils 18 is a conventional electron emitting material (not shown) such as BaO, SrO or CaO. Electrical connection for the lead-in conductor 16 is effected by contact pins 20 which project from supporting base caps 22 at either end of the lamp 10. The glass bulb 12 contains a filling of argon or other inert, ionizable gas in order to facilitate starting, as well as a small charge of mercury. In the operation of the lamp 10, when the electrodes 18 are electrically connected to a source of predetermined energizing potential via contact pins 20, a gas discharge is sustained between electrodes 18. The gas discharge generates ultraviolet radiation, a greater part of which has a wavelength of 254 nm and a comparatively small part of which has a wavelength of 185 nm. The glass bulb 12 has coated on its interior surface or inner wall a luminescent layer 26. The luminescent layer 26 is comprised of a phosphor material 28 such as (Ca 5 (Po 4 ) 3 (F,Cl): Sb, Mn), for example. A three-component mixture of blue, green and red emitting phosphor materials containing rare-earth metals may be used for the luminescent layer 26 emitting blue, green and red lights each having a relatively narrow band emission spectrum. Trivalent europium-activated yttrium oxide in accordance with the formula (Y 0.955 , Eu 0.045 ) 2 O 3 for red emission, bivalent europium-activated halophosphate phosphor material in accordance with formula (Sr 0.55 , Ba 0.35 , Eu 0.01 ) 10 (PO 4 ) 6 Cl 2 for blue emission and terbium-activated cerium containing phosphor material in accordance with formula (La 0.50 ,Ce 0.35 ,Tb 0.15 ) 2 O 3 · P 2 O 5 · 0.0005 B 2 O 3 for green emission are preferable. The phosphor material 28 includes fine phosphor particles or phosphors 30 on which a continuous layer 32 is formed as shown in FIG. 2. The layer (herein-after also called the protective layer) prevents ultraviolet at 185 nm from transmitting to the phosphors 30 therethrough. The protective layer 32 is composed of a metal oxide having more than 80 % spectral transmittance for ultraviolet at 254 nm and less than 50 % spectral transmittance for the ultraviolet at 185 nm. Metal oxides such as MgO, Y 2 O 3 , ZrO 2 and SrO are preferable for the protective layer 32, which transmit the ultraviolet well at 254 nm but effectively reflect or block the ultraviolet at 185 nm, however other metal oxide(s) such as La 2 O 3 , Sm 2 O 3 , Gd 2 O 3 , Dy 2 O 3 , Ho 2 O 3 , Er 2 O 3 , Yb 2 O 3 , Lu 2 O 3 , α-Al 2 O 3 and/or BeO are also used as the protective layer 32. The protective layer 32 is composed of at least one metal oxide selected from the group consisting of MgO, Y 2 O 3 , La 2 O 3 , Sm 2 O 3 , Gd 2 O 3 , Dy 2 O 3 , Ho 2 O 2 , Er 2 O 3 , Yb 2 O 3 , Lu 2 O 3 , ZrO 2 , SrO, α -Al 2 O 3 and BeO and the amount thereof in the phosphor material is in the range of 0.005 wt% to 3.0 wt%. FIG. 4 is a graph showing the spectral transmittance of metal oxide layers MgO, Y 2 O 3 , ZrO 2 and Ta 2 O 5 each having the average (mean) thickness of 1.4 µm, 0.7 µm, 0.2 µm and 0.2 µm, respectively. As can be seen in the graph, MgO, Y 2 O 3 , and ZrO 2 exhibit a high transmittance for ultraviolet having wave lengths of more than 254nm and a low transmittance for ultraviolet having wavelengths less than 220 nm. On the other hand Ta 2 O 5 exhibits a low transmittance for ultraviolet having wavelengths less than 380 nm. Metal oxide layers composed of Ta 2 O 5 are not recommended for the protective layer 32 because the conversion from the ultraviolet radiation at 254 nm to visible light is not effected by the phosphor 30. The intensity ratio of ultraviolet radiation at 185 nm to that at 254 nm in the popular 40 W type fluorescent lamp (0.0029 W/cm 2 ) having the bulb length of 1200 mm and the inner diameter of 37mm is about 0.12 while it is about 0.2 in the high output type fluorescent lamp (FLR60 D/A ) (0.043 W/cm 2 ) having the bulb length of 1200 mm and the inner diameter of 37mm. If the protective layer 32 is used for the high output type fluorescent lamp, it reduces the amount of the ultraviolet at 185 nm reaching the phosphor particles 30, which is equal to decrease the intensity ratio of the 185 nm radiation to the 254 nm radiation to 0.15 or less. For preliminary evaluation of the protective layer 32, three sample phosphor materials each having a different metal oxide layer of ZrO 2 , Y 2 O 3 or MgO continuously formed thereon were prepared. Those phosphors having formula SrB 4 O 7 : Eu were found to deteriorate more by the 185 nm radiation than phosphors of the formula Ca 5 (Po 4 ) 3 (F,Cl): (Sb, Mn) so that the effect of the sample phosphor material can be easily appreciated. The sample phosphor material was prepared by the following method. Firstly, the phosphor was dispersed into a solvent such as 500 ml ethanol. A metal alkoxide solution containing one metal selected from Zr, Y and Mg was added to the phosphor-dispersed solution to provide slurries. A predetermined amount of the metal was used so as to contain the metal oxide in the protctive layer of the phosphor material in the range of 0.005 to 3.0 wt%, such as 1 wt%. The slurries were kept in a vessel and stirred for a predetermined period at 70 °C to 90 °C to cause hydrolysis of the metal alkoxide solution. Polymerization of the metal oxide occurred on the surface of the phosphor particles. Excess alcohol solvent was evaporated and the polymerized metal oxide was dried and heated to obtain a continuous, metal oxide layer on the phosphor particles. Three test fluorescent lamps (Examples X, Y and Z) were also made using these sample phosphor materials. Each test fluroescent lamp (which had a glass bulb of inner diameter 13.5 mm and length 350. mm) was operated with a wall load of 0.007 W/cm 2 . Lumen maintenance rates of the test lamps at 100 hours, 1000 hours and 2000 hours were measured. The results of the measurement are shown in TABLE I together with lumen maintenance rates of two comparative fluorescent lamps. The two comparative lamps were Fabricated in a similar manner as the test lamps, but one of them had a continuous metal oxide layer formed from γ-Al 2 O 3 on the phosphor particles whereas the other comparative lamp had no such metal oxide layer formed on its phosphor particles. Metal Oxide (Hr) 0 100 1000 2000 EXAMPLE X ZrO 2 100% 92% 85% 76% EXAMPLE Y Y 2 O 3 100% 93% 88% 80% EXAMPLE Z MgO 100% 94% 82% 72% Comparative) No layer 100% 78% 61% 48% Comparative) γ-Al 2 O 3 100% 88% 77% 59% The lumen maintenance rate at 2000 hours for each test lamp according to the invention exhibited more than 70 % while it was less than 59 % for both comparative lamps. The metal oxide layer formed from ZrO 2 , Y 2 O 3 or MgO clearly improves the luminous efficacy because such metal oxide layer has been found to function in preventing ultraviolet radiation at 185 nm from transmitting therethrough to the phosphor particles, whilst they transmit ultraviolet radiation at 254 nm efficiently. The metal oxide layer itself has also been found to be less reactive to mercury whereby it also functions to prevent mercury from adhering to and reacting with the phosphor particles. The protective layer is not limited to the metal oxide selected from ZrO 2 , Y 2 O 3 and MgO, it may be at least one metal oxide selected from the group consisting of La 2 O 3 , Sm 2 O 3 , Gd 2 O 3 , Dy 2 O 3 , Ho 2 O 3 , Er 2 O 3 , Yb 2 O 3 , Lu 2 O 3 , SrO, α -Al 2 O 3 and BeO. A precise correspondence between the energy gap at the forbidden band of the metal oxides and the quantum energy of ultraviolet could not be obtained. However as shown in TABLE II it was substantially confirmed that a preferable protective layer 30 has the energy gap ranging from 5 electron volts to 8 electron volts at the forbidden band substantially corresponding to quantum energy of ultraviolet at 185 nm. It is assumed if a metal oxide layer has an energy gap less than 5 electron volts, ultraviolet at 254 nm is effectively absorbed by the metal oxide layer. It is also assumed if a metal oxide layer has an energy gap more than 8 electron volts, the ultraviolet at 185 nm is not absorbed by the metal oxide layer. Metal Oxide Energy Gap (eV) Metal Ion Electro-negativity MgO 7.3 6 Y 2 O 3 - 8.4 La 2 O 3 5.4 7.7 Sm 2 O 3 5.0 7.7 Gd 2 O 3 5.3 7.7 Dy 2 O 3 5 7.7 Ho 2 O 3 5.42 7.7 SrO 5.7 5 Er 2 O 3 5.42 7.7 Yb 2 O 3 5.22 7.7 Lu 2 O 3 5.5 7.7 α -Al 2 O 3 7.0 - BeO 5.2 7.5 ZnO 2.62 - TiO 2 3.7 - γ -Al 2 O 3 - 10.5 B 2 O 3 4.89 14 As examined the metal ion electronegativity Xi of the metal oxides, it was found that it is in a range of 5 and 9 as shown in TABLE II, wherein Xi = (1+2Z)X o in which Z represent the ion valency of a metal ion and X o represents the Pauling's electronegativity. Metal oxides having the metal ion electronegativity ranging from 5 to 9 exert less attractive force to mercury oxides. Metal ion electronegativity of CaO and BaO falls within the range. It has been observed that spectral transmittance of the metal oxide layer for ultraviolet at 254 nm falls when the thickness of the layer increases. When the metal oxide layer is used in an amount of more than 3 wt% to the phosphor material, the transmittance of the ultraviolet at 254 nm may deteriorate due to excessively formed metal oxide particles in the layer. Transparency of the layer may also deteriorate when the metal oxide layer is formed by using a metal alkoxide solution in which the metal oxide particles are left as the residue. When the metal oxide layer is used in an amount of less than 0.005 wt% to the phosphor material, the layer may not work effectively as a protective layer against ultraviolet at 185 nm and mercury. Preferred phosphors (or phosphor particles) which may be employed in accordance with the present invention are named below. General formula : a(M 1 , Eu)O · bAl 2 O 3 , wherein M 1 is at least one selected from Zn, Mg, Ca, Sr, Ba, Li, Rb and Cs and a>0, b>0, 0.2≦a/b≦1.5 (aluminate phosphors activated by bivalent europium) General formula : a(M 1 , Eu, Mn)O · bAl 2 O 3 , wherein M 1 is at least one selected from Zn, Mg, Ca, Sr, Ba, Li, Rb and Cs and a>0, b>0, 0.2≦a/b≦1.5 (aluminate phosphors activated by bivalent europium and manganese) General formula : (M 2 , Eu) 10 (PO 4 ) 6 · X 2 wherein M 2 is at least one selected from Mg, Ca, Sr and Ba and X is at least one selected from F, Cl and Br (halophosphate phosphors activated by bivalent europium) General formula : (Y 1-s , Eu s ) 2 O 3 , wherein 0.005≦s≦ 0.20 ( yttrium oxide phosphors activated by trivalent europium) General formula : (Y 1-s , Eu s ) 2 O 2 S, wherein 0.005≦s≦0.20 ( yttrium oxysulfide phosphors activated by trivalent europium) General formula : (Y 1-s , Eu s ) (P, V)O 4 , wherein 0.005<s<0.20 ( yttrium phospho-vanadate phosphors activated by trivalent europium) General formula : (RE 1-x-y , Tb x , Ce y ) 2 O 3 · cAl 2 O 3 bSiO 2 eP 2 O 5 wherein RE is at least one selected from Y, La and Gd and x>0, y>0, 0.1≦x+y≦0.07, c≧0, d≧0, e>0, 0.08≦c+d+e≦1.30 General formula : (RE 1-x-y , Tb x , Ce y ) 2 O 3 fSiO 2 gP 2 O 5 · hB 2 O 3 , wherein RE is at least one selected from Y, La and Gd and x>0, y>0, 0.1≦x+y≦0.07, f≧0, g>0, 5.0×10 -6 ≦h≦6.0×10 -3 , 0.8≦f+g+h≦1.30 General formula : (RE 1-t-w Tb t , Ce u ) 2 O 3 iM 3 O · j(Al B y ) 2 O 3 , wherein RE is at least one selected from Y, La and Gd , M is at least one selected from Zn, Mg, Ca, Sr and Ba and 0.5≦i≦4.0, 0.2≦j≦1.40, 0<t+u≦1.0, v<1.0×10 -4 Phosphor materials for applying in fluorescent lamps were prepared by the similar manner as explained above from the phosphors and the metal oxides shown in TABLE III and TABLE IV. Continuous layers of the metal oxides were formed on the phosphors (phosphor particles). The phosphor materials were applied in the fluorescent lamps as described with reference to FIG. 1. Applying the phosphor materials to the fluorescent lamp in order to form the luminescent layer on the inner wall of the bulb was carried out in a conventional manner, for example by mixing the phosphor materials with solvent, coating the tube wall with suspension of the phosphor materials, drying and heating or remove a binder. Five lamps were made for each example. In order to evaluate the fluorescent lamps thus made, two different types of fluorescent lamps were also provided. One type (comparison sample type A ) was the fluorescent lamp, which was provided with a luminescent layer made of the same phosphor used in the sample. Metal oxide layers on the phosphor particles were not formed in the comparison sample type A. The other type (comparison sample type B ) was the fluorescent lamp, which was provided with a luminescent layer made of the same phosphors and the metal oxides used in the sample. However, continuous metal oxide layers were not formed on the phosphor particles. Five lamps were made for each type. Measurement of the luminous output (lumen/W) of the lamps at 0 hour (after stabilizing of the lamp) and 1000 hours was carried out. Both of the initial luminous output and that after 1000 hours lighting are shown as relative values when each value of the comparison sample type A is expressed 100 %. The differences of the two extremes in the luminous output for both of the example and the comparison sample type B are shown as a deviation value in TABLE III and TABLE IV. Explanations on examples 1 to 3 are described below. Example 1 0.12 g of magnesium (metal) was dissolved in 100 cc of methoxyethanol [CH 3 OCH 2 CH 2 OH] to provide diethoxymagnesium solution [Mg(C 2 H 5 O) 2 .]18.5g of blue emitting phosphors representing the formula (Sr 0.55 , Ca 0.35 , Ba 0.01 , Eu 0.01 ) (PO 4 )Cl 2 , 40.0g of green emitting phosphors representing the formula (La 0.50 , Ce 0.35 , Tb 0.15 ) 2 O 3 · 0.985PO 5 · 0.0005B 2 O 3 and 1.5 g of red emitting phosphors representing the formula (Y 0.955 , Eu 0.045 ) 2 O 3 were added to the diethoxymagnesium solution under stirring to prepare slurries. The slurries werekept in a vessel at about 80 °C for about 1 hour to volatilize ethanol therefrom. Uniform layers of diethoxymagnesium were formed on the surfaces of the phosphor particles. The phosphors having such layers were heated at about 500 °C for about one hour to form magnesium oxide layers on the phosphor particles. The amount of the magnesium oxide layers uniformly formed on the phosphor particles was 0.2 wt%. Phosphor materials including the oxide layers formed on the phosphor particles thereof were thus obtained. The phosphor materials were applied on the glass bulb having a diameter of 15.5 mm. Each lamp was operated at 4 W (wall load, 0.11 W/cm 2 ). Example 2 1.21 g of trimethoxyyttrium [Y(OCH 3 ) 3 ] was dissolved in 150 cc of xylene to provide trimethoxyyttrium solution. 150g of red emitting phosphors representing the formula (Y 0.960 , Eu 0.040 ) 2 O 2 S were added to the trimethoxyyttrium solution under stirring to prepare slurries. The slurries were kept in a vessel at about 70 °C to volatilize methanol therefrom. Powdered substances were left in the vessel. The powdered substances were then heated at about 600 °C for about five hours. Phosphor materials including yttrium oxide layers formed on the phosphor particles thereof were finally obtained. The amount of the oxide layers was 0.5 wt%. The phosphor materials were applied on the glass bulb having a diameter of 15.5 mm. Each lamp was operated at 4 W (wall load, 0.11 W/cm 2 ). Example 3 0.139 g of dibutoxybarium [Ba 2 (OC 4 H 9 ) 2 ] was dissolved in 150 cc of butyl alcohol to provide dibutoxybarium solution. 150g of green emitting phosphors representing the formula (La 0.60 , Ce 0.25 , Te 0.15 ) 2 O 3 · P 2 O 5 was added to the dibutoxybarium solution under stirring to prepare slurries. The slurrieswere kept in a vessel at about 90 °C. Powdered substances were left in the vessel after evaporating the butyl alcohol. The powdered substances were then heated at about 600 °C for about one hour. Phosphor materials were finally obtained. The phosphor materials contained uniform layers of barium oxide formed on the surfaces of the phosphor particles. The amount of the oxide layers was 0.05 wt% . The phosphor materials were applied on the glass bulb having a diameter of 15.5 mm. Each lamp was operated at 4 W (wall load, 0.11 W/cm 2 ). Examples 4 to 14 Phosphor materials of the examples 4 to 14 were also prepared with the phosphors and metal oxides shown in TABLE III and TABLE IV in a similar manner as described above. Fluorescent lamps having such luminescent layers formed from the phosphor- material were operated for evaluation. The results of the evaluation are shown in TABLE III and TABLE IV. As can be seen in TABLE III and TABLE IV, each luminous output of the sample lamps 1-12 at 0 hours is almost the same as that of the comparison sample type A (having no metal oxide layer) and they are more than 100 % at 1000 hours. The deviation rates of the sample lamps 1 to 3 at 0 hours are within 1.5 and they are within 2.5 at 1000 hours. On the other hand, the deviation rates of the comparison sample type B at 0 and 1000 hours are about 3.0 and 5.0 toward negative direction, respectively. This means that the metal oxide layers continuously formed on the phosphor particles in the sample lamps 1 to 12 efficiently worked as protective layers against ultraviolet at 185 nm and mercury. The sample lamps 13 and 14 were not preferred examples. The luminous outputs of the sample lamp 13 at 0 and 1000 hours are the same as that of the A type comparison sample and the deviation rates at 0 and 1000 hours are the same as that of the B type comparison sample. Regardless of the metal oxide layers formed on the phosphor particles in the sample 13 they did not work effectively. The metal oxide layer of the sample lamps 13 was not formed continuously due to a shortage of the metal oxide (the amount of the metal oxide in these sample was 0.001 wt%) and they were no longer protective layers against ultraviolet at 185 nm and mercury. In the sample lamp 14, the initial luminous output was not improved due to an excess of the metal oxide, the amount of which was 4.0 wt%. The protective layer 32 on the phosphor particle 30 shown in FIG. 2 is formed with a single layer of metal oxides, however it may be multilayer. FIG. 3 shows a protective layer 36 consisting of a set of inner and outer layers 38, 40 formed on the phosphor particle 30. The inner layer 36 is composed of a metal oxide mainly for transmitting ultraviolet at 254 nm and visible light. The outer layer 40, which is formed with the metal oxide, surrounds the inner layer 38 for substantially preventing mercury from adhering to the phosphor particles 30. The protective layer according to the invention may contain additives for the purposes such as to have a good layer strength, good layer formability or good adhesion with the phosphor particle. The phosphor material is coated directly on the inner wall of the bulb of the fluorescent lamp as shown in FIG. 1, however it is coated on a transparent conductive film when rapid-start type fluorescent lamps are fabricated. The phosphor material for fluorescent lamps according to the invention may be formed either directly or indirectly on the inner wall of the fluorescent lamps. FIG. 5 shows a lamp fixture 44 in which the fluorescent lamp 10 described with FIG. 1 is fitted. In order for causing the discharge in the fluorescent lamp 10, electric energy is supplied to the electrodes 18 of the lamp from a lighting circuit 46 disposed in the lamp fixture 44. It is to be understood that the phosphor material according to the invention is not limited in application to the fluorescent lamp described above, it is also applicable to fluorescent lamps having various bulb shapes, electrodeless fluorescent lamps operated with a high frequency source or mercury discharge lamps containing a large amount of rare gas. Additional advantages and modifications will readily occur to those skilled in the art. Accordingly, various modifications may be without departing from the spirit or scope of the general inventive concept as defined by appended claims.

Claims: A luminescent material for a mercury discharge lamp comprising: a phosphor material including phosphor particles which emit a luminous flux upon excitation by ultraviolet radiation at 254 nm; and a continuous protective layer formed on the phosphor particles comprising: 0.005 to 3.0 wt% of at least one metal oxide selected from the group consisting of MgO, Y 2 O 3 , La 2 O 3 , Sm 2 O 3 , Gd 2 O 3 , Dy 2 O 3, Ho 2 O 3 , Er 2 O 3 , Yb 2 O 3 , Lu 2 O 3 , CaO, ZrO 2 , SrO, BaO, α -Al 2 O 3 and BeO, obtainable by a process in which the phosphor particles are dispersed in an ethanol, methoxyethanol, xylene or butyl alcohol solvent; a metal alkoxide solution of a metal from said group being added to form a slurry, stirring the slurry at 70°C to 90°C to cause hydrolysis of the metal alkoxide solution, excess solvent being evaporated whereby the metal oxide layer formed is dried and heated to form said continuous protective layer. A luminescent material as claimed in claim 1 wherein: the metal oxide of the metal oxide layer formed on the phosphor particles has an energy gap of 5 electron volts to 8 electron volts at the forbidden band and which is capable of absorbing ultraviolet radiation at 185 nm. A luminescent material according to claim 1 or 2, wherein a metal ion of the or each metal oxide has an electronegativity between 5 and 9. A luminescent material according to any preceding claim, wherein the phosphor material includes a rare earth metal or the phosphor is activated by a rare earth metal. A luminescent material according to any preceding claim wherein the continuous protective layer formed on the phosphor particles substantially prevents ultraviolet radiation at 185 nm from transmitting therethrough. A luminescent material according to claim 5 wherein the protective layer prevents ultraviolet radiation at 185 nm from transmitting to the phosphors therethrough. A luminescent material according to any preceding claim wherein the metal oxide layer transmit ultraviolet radiation at 254 nm but effectively reflect or block ultraviolet radiation at 185 nm. A luminescent material according to any preceding claim in which the protective layer consists of a set of inner and outer layers formed on the phosphor particles. A luminescent material as claimed in any preceding claim in which the protective layer is a multilayer. A mercury discharge lamp comprising: a light transmissive bulb; a luminescent layer including a phosphor material formed on the inner wall of the light transmissive bulb; a quantity of mercury and a quantity of one or more rare gases filled in the light transmissive tube; and means arranged for generating positive column discharge in the light transmissive bulb, wherein the phosphor material includes a continuous protective layer as defined in any one of claims 1 to 4. A lamp device comprising: a mercury discharge lamp including a light transmissive bulb, a luminescent layer including phosphor material formed on an inner wall of the light transmissive bulb, the phosphor material including phosphor particles, a quantity of mercury and a quantity of one or more rare gases contained within the light transmissive bulb tube and first means arranged for generating and sustaining positive column discharge in the light transmissive tube, wherein the phosphor material includes a continuous protective layer thereon as defined in any one of claims 1 to 4, and a lamp fixture for fixing the fluorescent lamp including electrical circuit means for supplying electric energy to the first means for generating and sustaining the positive column discharge.

Inventors: NAOKI SHOJI, TAMATANI MASAAKI, TERASHIMA KENJI, WATANABE AKIO, YANAGISAWA FUMIYASU, NAOKI, SHOJI, TAMATANI, MASAAKI, TERASHIMA, KENJI, WATANABE, AKIO, YANAGISAWA, FUMIYASU, NAOKI, SHOJI, C/O PRINC. OFFICE, TOSHIBA LIGHTING, TAMATANI, MASAAKI, C/O INTELLECTUAL PROPERTY DIV., TERASHIMA, KENJI, C/O INTELLECTUAL PROPERTY DIV., WATANABE, AKIO, C/O PRINCIPAL OFFICE, TOSHIBA, YANAGISAWA, FUMIYASU, C/O INTELLECTUAL PROPERTY D.
Applicants: TOSHIBA KK, TOSHIBA LIGHTING & TECHNOLOGY, KABUSHIKI KAISHA TOSHIBA, TOSHIBA LIGHTING & TECHNOLOGY CORPORATION
Application year: 1994
Main classification: C09K11
IPC codes: H01J61, C09K11